Present commercial apparatus has the capability of combining images in copying or printing. For example, a textural image can be combined with suitable graphics or pictorials to form a single image having both text and graphics and/or pictorials. Similarly, a single color original can be reproduced with certain portions highlighted by reproduction in a different color from the rest of the reproduction. A related capability breaks down a multicolor original into its component colors and reproduces that original.
In attaining all of the above results using electrostatic imaging, the most common commercial approach is to create two or more toner images on a single image member and then to tranfer them in registration to a receiving sheet to form a combined toner image.
In apparatus that function primarily as a single color, office copier or printer, the most common way of presenting a receiving sheet to a plurality of toner images is to feed the sheet back to the image member along a path normally used for duplex. One inversion is added or subtracted to the duplex path to present the side of the sheet already holding an image to receive a second image. With this system, registration of combined images is dependent upon accurate cross track, in-track and skew registration of the receiving sheet recirculation mechanism. Very high quality registration has not been obtainable with this system. Acceptable registration generally requires a more sophisticated receiving sheet handling mechanism then is used in an ordinary duplex path. Results are also adversely affected by repeated passes of the receiving sheet through a fuser.
The most commonly used approach for combining three or more single color images to make a high quality multicolor image is to attach the receiving sheet to a transfer drum and rotate it repeatedly through transfer relation with the image member to superpose the images on the receiving sheet. The transfer drum and its sheet attaching mechanism are quite expensive and represent difficult technology to accomplish. When successfully implemented, they provide high-quality registration of the images being combined. The results are sufficiently good that the expense of a transfer drum is sometimes undertaken to obtain highest quality in highlight color applications.
A number of references suggest that images can be combined by transferring them in registration to an intermediate transfer member, for example, a drum or a web from which they are transferred to a receiving sheet. See, for example, U.S. Pat. No. 4,712,906, Bothner et. al., issued Dec. 15, 1987, which describes a multicolor imaging approach in which ledger-size images are transferred to an intermediate drum or web having a circumference large enough to handle a ledger-size image with its long dimension in the in-track direction. Letter-size images are positioned with their short dimension in the in-track direction transferring two letter-size images in the same space as a single ledger-size image.
U.S. Pat. No. 4,714,939, Ahern et. al., issued Dec. 22, 1987, is representative of a number of references which show a method of making duplex copies in which a first image is transferred to an intermediate and a receiving sheet is fed between the intermediate and the original image member while images are transferred from the intermediate and the image member to opposite sides of the single sheet. See also, U.S. Pat. No. 4,688,925, Randall, issued Aug. 25, 1987.